1. Field of the Invention The present invention relates to a high capacity lithium ion secondary battery and a method for forming a negative electrode plate for the lithium ion secondary battery.
2. Description of the Prior Art
In recent years, as more and more portable or cordless electronics devices have been used, more and more small-size and light-weight secondary batteries with high energy density have been put into practice as power sources for driving such electronics devices. Advances has been made in this field not only in application of such secondary batteries to small-size consumer products, but also in the development of large-size secondary batteries which are required to have long lasting resistance and safety. Specifically, lithium ion secondary batteries have high voltage and high energy density and thus are expected as one of power sources for not only electronics devices but also electrical power storage systems and electrical automobiles.
Under the above-described circumstances, instead of a carbon material such as graphite which has been widely used as a negative electrode active material of a lithium ion secondary battery in a conventional manner, an active material, for example, silicon (Si), tin (Sn) or a compound (for example, oxide, alloy or the like) of silicon or tin, having a larger theoretical capacity than that of graphite is used to advance development of increasing a capacity of a battery.
However, as for such an active material, its characteristic that it inserts/deinserts lithium when being alloyed with lithium is utilized and the volume of the active material expands/shrinks to a large extent due to charge/discharge. Accordingly, there has been a problem pointed out in which through repetition of expansion/shrink, the active material is micronized and lifetime of a charge/discharge cycle is reduced.
To cope with the problem, Japanese Laid-Open Publication No. 2006-019309 disclose a method in which an active material including silicon or the like is coated by ceramic, metal or the like to suppress micronization of the active material.
However, there are cases where when an active material is coated by ceramic (for example, Al2O3, SiO2 or the like), conductivity of the active material is reduced and thus properties of a battery (for example, high rate property) is degraded. When an active material is coated by metal (Ni, Cu and the like), the problem of reduction in conductivity does not arise but fabrication process steps become complicated because plating, vapor deposition, CVD (chemical vapor deposition) and the like has to be used as a coating method.
To cope with this, Patent Laid-Open Publication No. 2002-279974 discloses a method in which an active material is formed in form of islands on a current collector, thereby suppressing micronization of the active material due to expansion/shrink at charge/discharge.
FIG. 6 illustrates respective steps for forming a negative electrode plate described in Patent Laid-Open Publication No. 2002-279974. As shown in FIG. 6, for example, a target 104 of single-crystalline Si is provided over a current collector 101 and Si is vapor deposited over the current collector 101 through a mesh 103 to form an active material 102 in form of islands.
According to this method, expansion/shrink of the active material 102 can be absorbed in spaces between island regions, so that micronization of the active material 102 can be suppressed. Accordingly, a stress applied to the current collector 101 due to expansion/shrink of the active material 102 is also reduced. Thus, the generation of wrinkles in the current collector 101 can be prevented and reduction in energy density can be suppressed.